1. Field of the Invention
The invention relates to pistons for internal combustion engines and including inserts, such as piston ring inserts and crown inserts, of a more resistant material than the material of the remainder of the piston.
Where a piston is made of a light metal such as aluminium or an aluminium alloy a piston ring cannot be accommodated in a piston ring groove formed directly on the light metal. This is because the piston rings are of an iron based material which would rapidly wear away the light metal. Accordingly, it is customary to include in the piston an annular piston ring insert which has a face flush with the skirt of the piston and which is made from an iron based material such as austenitic cast iron. A piston ring groove is formed in the insert and the insert, being of a harder material than the remainder of the piston, is better able to resist the wear from the piston ring.
In general, such inserts are located below the level of the crown so that, apart from the groove-carrying surface, they are surrounded by the material of the remainder of the piston. Thus, the piston ring insert is held securely in the piston body.
It has been proposed, however, to place the piston ring groove as close as possible to the crown. This, in turn, moves the piston ring as close as possible to the crown which has the advantage of reducing to a minimum the `dead space` extending around the skirt above the piston ring and below the level of the crown. This, in consequence, reduces the volume of fuel/air mixture which collects in this space and whose incomplete combustion reduces the efficiency of the engine.
In order to achieve this, it is necessary to move the piston ring insert as far towards the crown as possible. This is done by having the insert in a rabbet extending around the crown so that the one surface of the insert forms a peripheral portion of the surface of the crown. Thus, since the insert is no longer surrounded by the remainder of the piston, the problem exists of preventing the piston ring insert from separating from the remainder of the piston in an axial direction i.e. from slipping off the end of the piston.
2. Description of the Prior Art
One previous proposal for overcoming this problem has been to make the insert radially narrower at the crown end than at the end remote from the crown so that the insert sits in a rabbet whose radial surface lies in a plane generally normal to the piston axis and whose overall surfaces subtends an angle of less than 90.degree. with the radial surface. Thus there is a positive mechanical interlock between the insert and the remainder of the piston in an axial direction which prevents the insert slipping off. This has the diadvantage, however, that the narrowing path for heat conduction axially downwardly from the crown through the piston material, and the widening path for heat conduction axially downwardly from the crown through the insert, cause `hot spots` to develop on the crown which can damage the narrow annular edge formed between the axially extending surface of the rabbet and the crown surface.
Satisfactory heat conduction can be achieved if the radial and axial surfaces are generally normal to one another. It has been thought that, even in the absence of positive mechanical interlock between the insert and the piston in such an arrangement, the insert will be held in position by a metallurgical bond thought to exist at the interface between the insert and the piston. Recent experience has shown, however, that this bond, if it exists, is insufficient to prevent the insert slipping off.
In addition, light metal pistons are, for certain applications such as diesel engines, provided with a combustion bowl in the crown. The edge between the upper end of the bowl and the crown surface of the piston at the entrance to the bowl is thin and is subject to the adverse conditions encountered in the combustion chamber. Under these conditions, an edge of aluminium or aluminium alloy tends to crack and this is plainly undesirable.
In order to overcome this problem, it has been proposed to include in the crown an insert of a material more resistant to the conditions in the combustion chamber than the aluminium or aluminium alloy of the piston. Such an insert is so located that the edge between the bowl and the crown surface is formed by the insert. Previous proposals for securing such inserts into the crown have included a screw-threaded connection and the use of a retaining ring welded to a part which is integral with the body.
Both of these proposals have disadvantages. Screw-threaded engagement is difficult since the piston and the insert generally have differing coefficients of expansion and an aluminium or aluminium alloy thread is relatively weak. Where a welded retaining ring is used, it is necessary to cast a ferrous insert into the aluminium or aluminium alloy body and then to weld the ring to the ferrous insert. The casting of the ferrous insert is complicated and there are also difficulties in ensuring that the crown insert is compressed by the welded ring.